Vacuum arc remelting (VAR) processes are commonly used in the production of metal ingots as a secondary melting process. By employing VAR processes to metal ingots, the resultant ingot produced in the VAR process may have increased chemical and/or mechanical homogeneity, which is desirable for metals used in demanding industrial applications. Additionally, because the solidification time of metals during VAR processes can be much more rapid, use of VAR processes may allow for a greater control of microstructure characteristics of an ingot metal. Example metals that are commonly remelted via VAR processes may include, but are not limited to including, nickel, titanium, steel, and any alloys derived from such metals or other metals.
VAR processes may utilize a VAR furnace, which uses direct current (DC) electrical power to remelt metals within a vacuum chamber. A VAR furnace may include a melting chamber and a movable ram that is connected to a DC power supply. The metal to be remelted may begin the VAR process as an electrode, which is connected to the movable ram. The remelted metal may be remelted as an ingot in a water-cooled copper crucible, within the melting chamber. To provide an atmosphere which contains negligible oxygen content, which may react with the metal being melted, and to evacuate impurities from the melting chamber, VAR furnaces may include a vacuum source. Further, in some VAR furnaces, a cooling system is included to extract the heat from the melting chamber.
Control of VAR systems may be based on controlling the arc gap between the end of the electrode and a melt pool formed during remelt, atop the ingot and/or crucible. In VAR practice, it has been observed that keeping a relatively constant arc gap may aid in providing consistent remelt results during the VAR process. Accordingly, control of the VAR process may be, at least in part, based on controlling the arc gap. However, it is often impractical or impossible to physically view the arc gap during the process itself and, therefore, the arc gap may be determined or derived based on other data that is more accessible. For example, some control methods determined that length of the arc and the resistance of the arc (e.g., the voltage drop caused by the arc) have a correlation.
Further, during the remelting process, it has been observed that short-duration short circuits (e.g., a small number of milliseconds) occur during remelting. It has additionally been observed that the frequency of the short circuits has a correlation to the arc gap. Such information may be used to control and/or maintain the arc gap. In prior control methods, the power input has been chosen to obtain the desired melting rate while the velocity of the ram has been altered to dynamically control and/or maintain the arc gap.
However, during VAR processes wherein it is desired that the resultant ingot has a large diameter (e.g., greater than 750 millimeters), ram velocity control may be difficult and/or controlling said velocity may provide inaccurate control of the arc gap, due to the increasingly larger amount of metal that needs to be melted for the same change in arc gap, as the diameter of the electrode increases. Therefore, improved control systems for VAR processes, in which arc gap can be controlled independent of the adjustment of the ram velocity, are desired.